Power converters are used in a variety of portable electronic devices, including laptops, mobile devices, cellular phones, electronic digital pads, video cameras, digitals cameras, and the like. In addition, power converters may be used in non-portable applications, such as liquid-crystal display (LCD) backlighting, automotive lighting, and other general purpose or specialty lighting.
Power converters come in many forms. Some converters are DC-DC converters, which convert a Direct Current (DC) input voltage to a different DC output voltage. AC-AC converters convert one Alternating Current (AC) input voltage to a different AC output voltage. DC-AC converters convert a DC input voltage to an AC output voltage, and AC-DC converters convert an AC input voltage to a DC output voltage.
Conventional AC-DC power converters typically include a diode bridge rectifier stage (i.e., a bridge or full-wave rectifier) and a bulk storage capacitor. The incoming AC voltage is generally provided by an AC power supply or AC line, which is converted to a DC output voltage when run through the diode bridge rectifier and bulk storage capacitor. This DC voltage is typically further processed by a converter, which generates an output signal that is applied across a load.
In this configuration, the rectifying circuit only draws power from the AC line when the instantaneous AC voltage is greater than the voltage across the bulk storage capacitor, resulting in a non-sinusoidal current signal that has high harmonic frequencies. A drawback with this configuration is that the power factor or ratio of real power to apparent power is usually very low. Thus, the converter draws excess current but fails to use the excess current to perform or accomplish any circuit functions.
To address the power factor issue, it is common to couple a power factor correction (PFC) stage to the diode bridge rectifier, which improves the use of current drawn from the main AC line by shaping it to be more sinusoidal. Generally, power converters that include PFC stages are either double-stage or single-stage power converters.
A converter having a double-stage PFC architecture allows for optimization of each individual power stage. However, this type of two-stage architecture uses many components and processes the power twice.
A converter having a single-stage PFC architecture uses fewer components and processes the power one time, which can improve efficiency and can be more reliable than a double-stage PFC architecture. But, a major drawback with the single-stage architecture is that it has a large output current ripple, which is at twice the AC line frequency. The magnitude of this ripple can overdrive conventional feedback networks—forcing them outside of their linear response region or degrading their ability to maintain a high power factor.
One technique for smoothing out or decreasing the large output current ripple is to couple a filtering capacitor, having a large capacitance value, to the output filter network. However, although a filtering capacitor having a large capacitance value smoothes out the large output current ripple delivered to the load without interfering with the control loop, such a filtering capacitor is usually an electrolytic capacitor that tends to be large and expensive and tends to degrade circuit reliability.
In addition, the large capacitance of such a filtering capacitor slows the response time of the control loop—resulting in excessive current, which can overdrive, and potentially damage, the load. The excessive currents typically occur when the load is connected to a pre-powered converter (e.g., “hot plug”, “hot insertion”). The output capacitor at this point is fully charge to the maximal output voltage; thus, the energy stored in it can damage the load right at the connection of it to the converter.
As a solid state light source, LEDs are being used more and more frequently due to their superior longevity, low-maintenance requirements, and continuously-improving luminance. In low-power lighting applications, the cost of LED drivers that are used to power LED loads is a critical design consideration. Such costs, however, must also be weighed against the necessary performance criteria of LED drivers, which must not only be efficient but also generate minimal ripples in the output current provided to the LED load. Large current ripples reduce the reliability, longevity, and luminance output of the LEDs, which is obviously not desirable.
Although there are numerous LED driver designs that use either two-stage power converters or single-stage power converters, one common type of LED driver is a two-stage PFC converter that includes an active PFC stage followed by a DC-DC converter stage. The active PFC stage provides a near unity power factor and a low total harmonic distortion (THD) across the entire universal input voltage range, while the DC-DC stage is used to provide tight regulation and control on the current output provided to the LED load. The DC-DC converter stage may also be referred to as a downstream isolation and regulator circuit, since it is configured to receive the DC voltage and the DC current output by the active PFC stage and then to provide a substantially constant current to the load (or LED load).
The active PFC stage is typically accomplished with a boost power topology. A drawback of such conventional designs, however, is the fact that these two stages require two independently controlled power switches and two control circuits (or “controllers”). The two-stage design suffers from an increased component count and a higher-than-desired cost.
Although it would be cheaper to employ a passive PFC as the first stage, such topology architectures usually cannot provide the necessary efficiency required by energy regulations or minimal current ripples required by the LED load. Another drawback to such two stage LED driver designs is that each LED driver is typically configured for one specific output current level. For each application requiring a different output current, a different LED driver is typically necessary.
For these reasons, there is a need for an LED driver that uses an active PFC stage, while still achieving necessary efficiency and minimizing output current ripples required by the LED load.
There is a need in the industry to be able to use a buck (step-down) topology, as the active PFC stage, that functions in transition mode (also referred to as boundary conduction mode or critical conduction mode operation), using one of the many low cost control chips, or integrated circuits (ICs), that are typically only designed to be used with either flyback or boost topologies. However, such low cost ICs generally do not work well with buck topologies because they do not provide good THD results (i.e., they often draw more than desired —or required by law or regulation—power from the AC power supply).
Although there are some active and expensive control chips or ICs that have been developed specifically for use with buck topologies to improve their THD results, there remains a need in the industry for enabling buck topologies to be used with the above-mentioned, lower-cost control chips or ICs, while still being able to achieve good THD results. Further, there is a need in the industry for a single LED driver that can provide at least two different output currents, preferably switchable by the user, so that such single LED driver can be used with a wider range of LED load applications.